Digital-optical imaging systems can be characterized using their optical transfer function (OTF). The OTF can represent the transformation of the spatial frequency content of the object onto the image plane. The OTF may be decomposed into a magnitude component, represented by the modulation transfer function (MTF) and a phase component, represented by the phase transfer function (PTF). The MTF can represent the contrast reduction at each spatial frequency, whereas the PTF can represent the spatial shift of the spatial frequency. The MTF and the PTF can be used to characterize imaging systems including imaging performance and image quality.